This invention relates to a nickel based hardfacing alloy and to a method of hardfacing substrates.
"Hardfacing" is a technique which involves applying a layer of hard material to a substrate for the purpose of increasing the wear and corrosion resistance of the substrate. The use of this technique has increased significantly over the years as industry has come to recognize that substrates of softer, lower cost material can be hardfaced to have the same wear and corrosion resistance characteristics as more expensive substrates of a harder material.
Hardfacing involves the deposition of a hard layer by welding or thermal spraying. Conventional weld hardfacing is accomplished by oxyfuel welding (OFW), gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), shielded metal arc welding (SMAW) and flux-cored arc welding (FCAW). Plasma transferred arc (PTA) hardfacing and laser beam hardfacing are also employed.
Substrates such as plastic extruder screw flights have been hardfaced with a variety of Ni based and Co based alloys to combat abrasive wear due to various plastic fillers and due to metal-to-metal wear between the extruder screw flights and hard surfaces with which they are in contact in service. To reduce abrasive wear, it is desirable to use hardfacing materials which have relatively low coefficients of friction. It is also desirable to use hardfacing materials having good corrosion resistance in view of acid release during placticizing. Such materials should also have good weldability and crack resistance.
Hardfacing materials have also included High Velocity Oxygen Fuel Thermal Spray process-fused coatings of Ni--Co--B--Si--WC, which have good abrasive wear characteristics. But such coatings are disadvantageously limited to a thickness of less than about 0.01 inch, and their relatively high hardness results in wear to mating material.
Hardfacing materials have further included PTA-applied Ni--Cr--B--Si alloys and Co--Cr--W--C alloys. The Ni--Cr--B--Si alloys have good abrasive wear resistance, but are crack sensitive at hardnesses greater than about Rc-50 and provide insufficient metal-to-metal wear resistance. The Co--Cr--W--C alloys have good adhesive wear resistance and weldability, but are relatively expensive and have relatively poor abrasive wear resistance.
Still further hardfacing materials have included nitrided low alloy or precipitation hardenable steels, which have good wear characteristics but suffer in the areas of thickness limitation, corrosion resistance, and repair weldability.
Relatively small extruder screws (less than about 1.5 in. diameter) have traditionally not been hardfaced but manufactured from proprietary tool steels such as CPM-9V and CPM-10V, but due to their relatively poor toughness, such steels are not suited for use on relatively large substrates, such as screws having a length of greater than about 1.5 inches.